Process and device for the application of elasticimetry, particularly of photoelasticimetry



Sept. 17, 1935. G, MABBOUX 2,014,688

PROCESS AND DEVICE FOR THE APPLICATION OF ELASTICIMETRY,

PARTICULARLY OF PHOTOELASTICIMETRY Filed Nov. 5, 1951 4 Sheets-Sheet 1 en-{ a W; ziou-a d ffer? G. MABBOUX 2,014,688 PROCESS AND DEVICE FOR THE APPLICATION OF ELASTICIMETRY,

' PARTICULARLY OF PHOTOELASTICIMETRY 5, 1931 4 Sheets-Sheet 2 Fi led Nov.

Sept. 17, 1935; G. MABBOUX PROCESS AND DEVICE FOR THE APPLICATION OF ELASTICIMETRY,

ULARLY OF PI CIMETRY PARTIC iO'I ELASTI Filed Nov. 5, 31 4 Sheets-Sheet 3 Sept. 17, 1935.

G. MABEOUX ,014,688 PROCESS. AND DEVICE FOR THE APPLICATION OF ELASTICIMETRY, PARTICULARLY OF PHOTOELASTICIMETRY Filed NOV. 1931 4 Sheets-Sheet; 4

5 020 3 Q [5] O [a] O Patented Sept. 17, 1935 UNITED vSTATES PROCESS AND DEVICE FOR THE APPLICA- TION F ELASTICIMETRY, PARTICULAR- LY 0F PHOTOELASTICIDIETBY Georges Mabboux, Paris, France vApplication November 5, 1931, Serial No. 573,216

' In Belgium November 10, 1930 11 Claims. The invention "relates to processes and devices for the measurement of strains which opaque bodies undergo.

Its main objectis to give these processes and devices, whatever they be, a more general application, whilst being simpler to operate and more efiichcious.

The invention will, at all events, be well understood with the aid of the following description as well as of the accompanying drawings, which description and drawings are, of course, given only by way of example.

Fig. 1 shows the optical diagram of an instrument made in accordancewith the invention.

Fig. 2 shows a detail of this instrument on a larger scale and in elevation.

Fig. 3 and-4 show the same instrument in side viewand in elevation respectively.

Figs. 5, 6 and 7 respectively show in elevation, three difierent arrangements in which the responsive elements are fixed on the surface of the body, the strains of which it is desiredto de termine.

Figs. 8 and 9 respectively show two modifications of the arrangement according to Fig. 5.

Figs. 10 and 11 respectively show, in longitudinal section, two difierent arrangements in .which responsive elements are fixed in the mass of the body, the strains of which it is desired to determine. I

Figs. 12 and 13 show, in longitudinal section and in transverse section on the line l3-l3 of Fig. 12, respectively a modification of the arrangement shown in Fig. '11.

Figs. 14, 15"and 16 respectively show,.in longltudinal section, three dilferent elements adapted' to be fixed in a work in concrete or in masonry.

Figs. 17, 18 and 19 respectively show-,"in elevation, three dilferent elements adapted to be inan iron work of art.

Figs.20and21show ferent responsive elemenis.

Fig. 22, finally, shows, in a diagrammatic side view, ameasuring instrument applicable in this latter case.

= fically CWO dif- The work of art in given, for ex- I ample at the positions in which the mechanical strains are the most important or most danger- The connection between theseelements and the 'material of which the work is composed should -bemadeastrustworthyaspossible. Onthe other hand, the in the surrounding-state of which result therefrom ou's, elements which are elastically responsive' are reduced to a minimum by giving the said elements very small dimensions in relation to the portion of the work to be studied.

The elastically responsive elements may be formed by deformable reflecting surfaces and is their deformations under the action of the strains are observed.

The elastically responsive elements may also be made of a. transparent material, for example of borosilicated glass, bakelite, celluloid etc. These elements may be lodged in a passage crossing the object to be studied from one side to the other, thus permitting the observation to be made by transparency. In the case in which this presents inconveniences or impossibllities, thisis is remedied by coacting the transparent element with a reflecting surface which enables the 011- I servationto be made by reflection. r

The responsive elements hereinbefore mentioned may be fixed to the surface of the ob- 20 ject to be studied or lodged in the mass constituting this object, a direction of access being then kept for them.

In many cases it may be admitted that the strains, indicated by changes in the shape 01'25 in the physical properties (double refraction) of the responsive elements, are related by simple functions to the forces acting upon the object to be studied. In the directions in which the principal strains (these directions always 'forming a right-angled system between themselves) act are known approximately, the responsive elements may be arranged in such a manner that each of them is subjected to only one ofvthese strains (by compression or tension) to the exclusion of the others.

The apparatus which enables measurement to be made of the changes undergone, under the action' of the strains, by the responsive elements fixed at difierent points of the work, must neces- 40 sarily be constituted by an independent set, which is easily transportable and manageable and easy to set in difierent directions. It should. moreover, be able to be supported on the work in order that one should be able to take the diiferent observations with ease whatever he the position in which the element to be studied is located. I

Such an apparatus is shown, byway ofexam- "ple, ,by Figs. 1 to 4, this apparatus. being more especially adapted for trt elements coated with a. reflecting surface. V

It comprises a stand I, the regulatable points of support of which are preferably located in a plane that is perpendicular to the axis of an observing instrument 2. In the latter there are fixed an eyepiece I, an analyzer .4 formed by a Glazebrook prism fox-example, a mirror without silvering or Brewster plate and a compensating dynamometer i. The set thus constituted can be brought in front of a mirror I embedded in the material 8 of the work to be studied. This mirror 'is illuminated with polarized lightobtained, by

,distributed by a diffusing screen ll.

reflection from the plate 5, of a beam of light emanating from a source 9 advantageously fed by a dry battery ll vof small dimensions. The beam emanating from this source can initially be The rays sent out by the mirror I again traverse the transparent material that it comprises and they may, after passing through the analyzer, be observed through the eyepiece 3. In the case in which it is desired to have a very powerful illumination, there may be fixed, in the path of the light rays, lenses or condensers such as l 2 and I: which pro duce an image of the source in the plane of the eyepiece. The compensator 6 is formed by a transparent screen of the-same material as the mirror I and acted upon, by tension or by compression with the aid of a calibrated spring I I and the screw IS with a graduated barrel, in a .direction in which one arrives, by acting on the -said screw l5, at compensating the double refrac- This enables the tion effect of the mirror 1. strain to which the mirror I is subjected to be measured by referring to the the screw l5.

The observation can be made by. having. recourse to a sensitive tint? plate, for example by -,,placing, between the analyzer 4 and the Brewster mirror 5, a transparent screen l6,-of mica for example, of suitable thickness and set in a suitable direction. This plate is engaged in a slide It, which can also serve as a mount for any other polariscopic accessory. In this case it is advantageous to have recourse to a mark of sensitive tint fixed in front of the compensator and in the path of the polarized light emanating from the plate 5, this mark being formed, as shown in Fig. 2, by a band I! of reflecting material covering only a part of the field and supported by a transparent screen I1 and this band sending to the observer the sensitive tint independently of the strains to which the mirror 1 on the one hand and the compensator i on the other hand are subjected.

In the case in which the double. refraction effects are small or determinable with d ifilculty with the aid of a compensator of the kind indicated above, the latter may be replaced by a com parator formed by a transparent plate coated with V .a reflecting surface and behaving in the same manner as the mirror I. This comparator is subjected to the actions of a dynamometer similar tothe one indicated above and enables the polari- .scopic measurement to be effected by comparison of the double refraction effects observed in the said mirror I and in the said plate or by comparison between the decreasing tint of the strained element with the increasing tint of the comparison plate acted'upon solely by the dynamometer.

' In the case in which the strains manifest themselves in a vertical plane, the optical system may comprise'a plummet enabling the setting of the planes of polarization of the optical instrument, in relation to the vertical, to be marked at the proper instant. As far as concerns thesetting in a horizontal plane, this can easily be determined with the aid of a compas or the like also provided graduated scale onprincipal strains. 5

Measurements carried out in this way can give immediately-the value of the difference of the principal strains of the mirror but thesum of these cannot be obtained except by laborious processes. It is therefore advantageous to'give 16 one of the principal strains a,known value, for example zero. This result is obtained by giving the mirror I a form that comp'risesa substantially rectangular region, two opposite sidesof this region being acted upon by compression (Fig. 5) or by tension (Fig. 6) whilst the two other sides are free.

In the case in which the direction of the prin-. cipal strains is known, a rectangular mirror I (Fig. 5) may be arranged so that its free sides are 20 parallel to the compression component. There can likewise be arranged nearthis responsive element a mirror 1' in the form of a tension gauge (Fig. 6), the longitudinal sides of which are free. These two mirrors thus enable the two principal strains prevailing in a certain place in the work tobe measured.

A circular mirror I (Fig. 7)- enables the respect-ive directions of the principal strains to be found when these directions are totally unknown at the beginning. It also enables the valueof the strains to be found in some cases in which the problem is particularly simple. It is, besides, advantageous, to fix such a circular mirror 1 near the mirrors I or P shown in Figs. 5 and 6. This 35 combination, on the one hand, enables the setting of the principal axes of the elastic ellipse to be known, and, on the other hand,-enables the size of the two diameters of the ellipse parallel to the direction of setting of the responsive element to 40 be determined, the ellipse being thus entirely;

ascertained. f I

The reflecting part of the niirror 1 may beout of contact with themass to be studied, a free space being left behind the said-part or an elastic or very compressible pad, such as 1shown in Figs. 10 to 12, being inserted.

When the difference between the coefilcients of elasticity of the materials forming the mirror and the mass acting upon it respectively is deemed to be too great, the responsive element can be givenin a.given direction a mean coefficient of elasticity, chosen arbitrarily between certain limits,,by having recourse to the device shown in Figs. 8 and 9, in which themirror 1 is finished offby shoulders II which distribute the reaction over a larger surface. Amovable joint II fixed between one of these shoulders and the mirror I avoids bendingmoments. If the whole is consolidated by being encircled by a ring I! (Fig. 9) it may be given a transverse coefllcient ofelasticity, different from zero. when it is desired to measure the strains not on the surface but within the mass itself of the 'bodytobe studied,-recourse maybehadto amirror I placed transversely at the bottom of a hole or passage 2| (Fig. 10).- In order to reduce the disturbance of the equilibrium to a minimum, the empty space thus created may be filled with a transparent material, such as Pokels flint, in- 7 sensible to the strains. The wall of the said hole or passage may. alternatively be reinforced by a tube 2|, of steel for example, the thickness of which is best chosen in accordance with the'relationship between the elastic coefficients of the materials present. In the case of structures of concrete or of iron, one of the reinforcements or portion of the ironwork, generally formed by a solid metallic rod, may be replaced by a tube in which the responsive element is fixed.

' In order to ascertain the. tensions in different planes that are not perpendicular to the axis of the said passage 20, recourse may be had to an auxiliary reflecting element which is insensible to the forces. This element may, for example, be formed by a mirrormade of inoxidizable steel, a total reflection prism 22 made of Pokels flint (Fig. 11) etc. It may be made a flxture'opposite the mirror 1 or may be introduced into the passage when taking the measurement, by being flxed to the end of a tube 21 which can be joined to the part 2 of the measuring instrument (Figs. 1

to 4). There may likewise be fixed at the bottom of the passage2ll, a plurality of differently set mirrors or even a cylindrical mirror 1* (Figs. 12 and 13) formed by a piece of transparent tube which is observed through a cylindrical lens 2| carried by the tube 23 oposite the reflecting element. The hole or passage 20 may be closed by a stopper 25 when it is not in use. In the in-- dustrial or experimental works, one may, in practice, meet with difliculties in obtaining an intimate connection between the mirrors I and the plastic materials (concrete, cement etc.) forming the work to be studied. It is, therefore, preferable to prepare, in the. laboratory, blocks 25 of ample dimensions in which the responsive elements or mirrors I are embedded. These blocks have on their external surface corrugations or deep striations 21 in order to ensure a good union with the work (Figs. 14 and 15). This union is "further accentuated by the shape of the blocks in the case of elementsacted upon by tension (Fig. 16). When the work is being constructed,

these blocks are flxed by simple means (wires 2' in Fig. 14, removable plates in F3. 15) to the framing 2! with the interposition of sheets 30 of paper or cardboard in order to prevent the deterioration of the elastically responsive elements I.

To make allowance for the efl'ects of the shrinkage of the cement and of the variations in the temperature, test pieces may be made of concrete of the same composition as the work to be studied and of .suitable'dimensions, and responsive elements be fixed in these pieces in the same manner as in thework and adapted to be permanently subjected to substantially identical actions.

For experiments of long duration, the silvering of the mirrors may be replaced by a platinizavance with their elastically responsive elements I, v

" In the case of metallic works, there may be permanently inserted, in the structure, elastic members such as Ilor 32 (Fig. 17 or 18) having responsive elements I, these members being arranged so as to havethe necessary freedom at the places where they are joined to the structure; this may be obtained by ensuring their union by the intermediary or bolts :3 or the like. These members are, besides, flxed so as ,to act with a reduced force on the responsive elements, i. e.,

they themselves bear nearly the whole of the actions of compression (Fig. 17) or of tension (Fig.

Fig. 19 shows an interesting application of the 5 invention to the study of strains which may be undergone by the expansion and articulation rollers 34 of supports 25 of structures such as bridges. In this case it is suflicient to make axially in each or some of the said rollers 34 ma passage 36 serving to lodge a responsive element of the kind indicated above. It is to-be noted that in this case the interpretation of the measurement made is particularly simple. This arrangement also constitutes an example of. the 15 case in which the observation takes place by transparency without leading to any appreciable disturbance of the stability of the whole. Recourse may also be had, according to the invention, to application of the simpler processes 20 by fixing, in the place of responsive elements I, slightly curved (Fig. 20) or concave (Fig, 21) or convex. reflecting elements, which, under the action of strains, undergo deformations or, more especially, changes in their curvatures. The 25 measurement of these changes enables the actions coming into play to be determined.

In Fig. 20, the reflecting responsive element is formed by a metallic plate 3! suitably set and fixed between two pieces 3', embedded in the 0' mass to be studied, in such a manner that the deformations of the said plate under the action of the strains will in no way be hindered. In Fig. 21, the responsive element is formed. by a block 39, the active reflecting surface 31 of which 35 is concave (or convex), this block being directly subjected to the action of the strains. The responsive elements may also be formed by discs with a spherical surface or by areas of the material to 'be studied which have been rendered 40 reflecting. The latter case is especially advantageous in cases of machine members or test pieces;

cable to this case. It comprises essentially a supporting stand ill, a source of light I provided with a condenser 42 and a diaphragm 43 the reticule of which is preferably cruciform. The beam emanating from'this source is made parallel by its passage through an objective lens II and directed in'the direction of the central normal to the reflecting surface 31 by the intermediary of a transparent mirror 45. The reflecting sarface' that is supposed to be concave converts this 7 parallel beam into a convergent, generally astigmatic, beam and this beam gives images of the reticule 43 which images can be observed through the eyepiece 4. In the focal plane of this eyepiece a likewise cruciform. reticule 41' is placed.

Two scales, one ll, for angles, in the plane of the co reticule and the other ll, linear, in the direction 0f the axis of the telescope, enable the characteristics of the astigmatic convergent beam to be determined and from these-it is eas'yto reduce the deformations of the mirror and, consequent- 5 ly, the strains to which the latter isbeing subjected.

If the reflecting surface is supposed to be con vex, the optics of the apparatus are changed to enable virtual images due to the divergent beam to be observed.

In the case in which itis desired tosubstitute, for the virtual observation, photographic or even chrono-phoiographic registration, it 'is sufllcient to replace the eye by a siitable photographic or r chrono-photographic apparatus. Chrono-photography can also be carried out with the aid of an intermittent source of light.

Stroboscopy can also flnd its application to the 5 above indicated methods, especially in the case of the study 01 objects subjected to vibratory strains. It is then suflicient to have recourse to a stroboscopic apparatus flxed between the instrument and the eye or more simply, to a source I of light-which is itself stroboscopic (neon lamps,

for example). I

It has hereinbefore been supposed that the observations are made on the normal scale and the distance of normal sight. It is obvious that 15 there-may be combinedwith the methods and with the apparatus indicated above, means enabling:

A microscopic observation to be made, for ex- ,ample, in the case of the study of the distribu- 20 'tion of 'the strains-and of the deformations between the elements of amicro-metallographic preparation or in-the case of the observation of stationary vibrations .of short wave length in the bodies subjected to examination, or an ob- 25 servation at 1 great distance, especially by telescopic means, for example in the case'oi the study of variations oi strains during the experimental destruction of a .work; in which case the observer and the measuring instrument should be free 30 from the danger that may result from this destruction or in the case in which observation at a short distance has become impracticable or diiflcult.

As a result of this, means are obtained which 35 allow to be measured in a simple, convenient and eflicacious manner, the strains acting upon various parts of a body and, more especially, of

an artiflcial structure, subjected to static and dynamic forces-during its setting up, on its be- 40 ing flnished, and for long years after being putto use. They also render possible an expert report to be made, in'the case where the work would cause anxiety, with a minimum of expense because the measuring apparatus is not 5 joined to the responsive elements and the latter are of a not very high cost price. It is to be noted that the presence of these elements does not appreciably disturb theelastic-equilibrium of the whole, sin'cea mirrorwith a surface 01' about one 50 square millimeter is suflicient for exact measuremerits.

. What I claim is:- g v 1. The method of measuring stresses in a structure comprising the steps of embedding a light 55 reflecting element, having double refractive properties when subjected to stresses, in said structure so that stresses in the latter arecommunicated to said element, said element having at least one surface exposed, projecting a beam of polarized e0 light'on said element, and measuring modifications in the reflected polarized light beam due to vthe double refractive effect 01' said element, whereby stresses in saidstructure communicated to said element may be detected and measured. 05 2. The method of measuring the stresses in -a structure comprising the steps offinserting a reflecting element, having a curved surface and double'rei'ractive properties when subjected to stresses, in said structure so that stresses in said 70. structure are communicated to said element, projecting a light beam on the curved surface of said element, and measuring the angle of rotation of the plane of polarization of the light beam. reflected from said element, whereby 7| stresses in said structure communicated to said element may be detected, measured and compared.

3; The method 01 measuring stresses in a structurecomprising the steps of embedding a reflecting element, having double refractive prop- 5' erties when subjected tostresses, in said structure so that stresses in the latter are communicated to said element, said element having at least one surface exposed, subjecting said structure to stress, projecting light on said element, and meas- 10 uring the angle oi refraction of light reflected from said element in'at least two diflerent planes, whereby stresses in said object communicated to said element may be detected and measured both as to quantity and direction.

4. In combination with an object subjected to stress, a light reflecting element immovably embedded in said object and having double refractive properties when subjected to stresses, said reflecting element being elastically responsive to the stresses in saidobject, means for projecting polarized light on said reflecting element, and means iormeasuring the eflect of said elementon the light.

5.-In combination with a structure subjected to stresses and having a cylindrical opening formed therein, a light permeable tube formed of material having double refractive properties when subjected to stress mounted in said cylindrical opening and in contact with the walls thereof, the outer surface of said tube being light reflective, means for projecting a beam of po+ larized light onto the inner wall of said tube, and

means for measuring the effect or said tube on the polarized light passed therethrough.

6. A method of measuring stresses in a structure comprising embedding in said structure, to receive stresses set up therein, a transparent element the double'refractive property of which varies relative to the.stresses applied thereto,

passing a beam cl polarized light through said element, reflecting the beam back through said element, and determining the eflect of the element on said beam of polarized light.

7.. A method of measuring stresses in a structure comprising providing a bore in said structure, positioning in said bore a transparent element, the double refractive property of which variesrelative to stresses applied thereto, providing contact between opposite sides or said ele- 5 ment and the wall of said bore to transmit stresses in said structure, in a direction parallel to 'a line passing through the points of contact of said element with said bore, to said element, passing a beam of polarized light through said element,

reflecting the beam back through said element,

and determining the eirect of the element on said beam of polarized light. I 8. A method of measuring stressee'in a structure comprising providing a bore in said strucg0 'ture, positionin in said bore a transparent element, the double-refractive property of which varies relative to stresses applied thereto, providing contact between opposite sides oiv said element and the wall 0! said bore-to transmit 5 ture comprising providing a bore in said structure, positioning in. said bore a transparent element, the double refractive property of which .varies relative to stresses applied thereto, pro- 5 viding-contact betweenopposite sides of said element and the wall or said bore to transmit stresses in said structure, in a direction parallel to a line passing through the points of contact of said elementfwith' said'bore, to said element, '1 positioning a second element in: another bore in the structure to respond to stresses atsubstan-V 'tially 90 to those received by theflrst element,

passing polarized light through said elements, re-

I flecting the polarized light back through each 15 element, and measuring the effect of the elements on the polarized light.

10. In combination with a structure subject to stresses having a recess formed therein, a trans- !parentelement positioned therein and exhibiting '20 double refraction when subjected to stresses, said element being in contact with the walls of said recess to receive stresses in said structure, means for passing polarized light through said element,

a reflecting surfacepositioned in said recess to receive light 'passed through said element and 'refleeting the same back through the element, and means for measuring'the efliect of said element on the polarized light. h j 11. In combination with a structure subjected to stresses, a transparent element'formed oi! ma- 10 terial having double refractive properties when subjected to stresses embedded in said structure 

